Thorium-Organic Framework Constructed with a Semirigid Triazine Hexacarboxylic Acid Ligand: Unique Structure with Thorium Oxide Wheel Clusters and Iodine Adsorption Behavior

Article abstract of DOI:10.1021/acs.inorgchem.9b03639

We successfully prepared and characterized a distinctive thorium-based MOF (metal-organic framework) Th-TTHA with thorium oxide wheel clusters under the conditions of solvothermal synthesis by utilizing thorium nitrate and semirigid triazine hexacarboxylic acid linker H₆TTHA (H₆TTHA = 1,3,5-triazine-2,4,6-triamine hexaacetic acid). To the best of our knowledge, Th-TTHA is the first thorium-based MOF assembled by semirigid triazine hexapod ligand H₆TTHA. It is worth mentioning that Th-TTHA exhibits a novel and distinctive arrangement of structure and topology. Th-TTHA has abundant adsorption sites such as the triazine ring that is rich in nitrogen atoms, N of NH-, and carboxyl oxygen atoms without coordination with a central metal, which drove us to explore its iodine adsorption capacity. The experimental results show that Th-TTHA shows excellent adsorption capacity for iodine, and the adsorption amount in a saturated iodine cyclohexane solution can reach 528 mg/g within 24 h. This work is greatly significant for the development of new structures of thorium-based MOFs and the expansion of its functional characteristics, which is very essential for our in-depth understanding of thorium chemistry and in the management and disposal of radionuclides and application of the nuclear fuel cycle.

Full text of DOI:10.1021/acs.inorgchem.9b03639

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Article
Thorium−Organic Framework Constructed with a Semirigid Triazine
Hexacarboxylic Acid Ligand: Unique Structure with Thorium Oxide
Wheel Clusters and Iodine Adsorption Behavior
Na Zhang, Li-Xian Sun, Feng-Ying Bai,* and Yong-Heng Xing*
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ABSTRACT: We successfully prepared and characterized a
distinctive thorium-based MOF (metal−organic framework) Th-
TTHA with thorium oxide wheel clusters under the conditions of
solvothermal synthesis by utilizing thorium nitrate and semirigid
triazine hexacarboxylic acid linker H6TTHA (H6TTHA = 1,3,5-
triazine-2,4,6-triamine hexaacetic acid). To the best of our
knowledge, Th-TTHA is the first thorium-based MOF assembled
by semirigid triazine hexapod ligand H6TTHA. It is worth
mentioning that Th-TTHA exhibits a novel and distinctive
arrangement of structure and topology. Th-TTHA has abundant
adsorption sites such as the triazine ring that is rich in nitrogen
atoms, N of NH-, and carboxyl oxygen atoms without coordination
with a central metal, which drove us to explore its iodine adsorption capacity. The experimental results show that Th-TTHA shows
excellent adsorption capacity for iodine, and the adsorption amount in a saturated iodine cyclohexane solution can reach 528 mg/g
within 24 h. This work is greatly significant for the development of new structures of thorium-based MOFs and the expansion of its
functional characteristics, which is very essential for our in-depth understanding of thorium chemistry and in the management and
disposal of radionuclides and application of the nuclear fuel cycle.
reported a uranyl organic framework FJI−H−U1 formulated
with [(CH3)2NH2]4[(UO2)4(TCPE)3]·(solvent)x with tbo
topological structure by tetrakis(4-carboxyphenyl)ethylene
(H4TCPE), which showed excellent selective adsorption
performance for dyes with positive charge.27 Wang et al.
reported a new and original thorium-based MOF
[Th3(bptc)3O(H2O)3.78]Cl·(C5H14N3Cl)·8H2O (SCU-8;
H3bptc = [1,1′-biphenyl]-3,4′,5-tricarboxylic acid) with
mesoporous structures, large surface areas, and superior
anion-exchange properties.10 In our previous work, our group
used triazine-tricarboxylic acid ligand H3TATAB (H3TATAB
= 4,4′,4″-s-triazine-1,3,5-triyltri-p-aminobenzoate) to construct
a distinctive uranyl organic framework U-TATAB
[(CH3)2NH2][UO2(TATAB)]·2DMF·4H2O (DMF = N,N-
dimethylformamide) and a thorium-based complex Th-
TATAB Th6O4(OH)4(H2O)6(TATAB)2(HCO2)6, which
showed outstanding adsorption performance for iodine and
dyes.8,14 Farha et al. used the tetrapodal linker 1,2,4,5-
INTRODUCTION
■
MOF materials are new types of metal−organic framework
(MOF) complexes, which are porous materials with infinite
structures constructed by metal ions and organic linkers
connected by coordination bonds.1−3 There are many kinds of
MOF materials, among which the material with a pore
diameter of <2 nm belongs to microporous material, and the
material with a pore diameter between 2 and 50 nm belongs to
mesoporous material.3,4 MOFs have become one of the most
attractive and important research directions in the realm of
chemistry because of its promising application in sensing,5−7
adsorption and removal of organic pollutants,8−10 gas storage
and separation,11−13 catalysis,14,15 bioimaging,16−18 drug
delivery,18,19 and the capture of water from the atmosphere.20
Compared with the MOF constructed by transition metals and
rare earth ions, the study of An-MOF is relatively rare. The in-
depth understanding of actinide chemistry is essential in the
management and disposal of radionuclides and application of
the nuclear fuel cycle. It is essential to develop novel and
diverse actinide-containing MOF structures. A MOF has been
used as an outstanding platform to explore and research the
behavior of actinide elements due to its advantages of synthesis
diversity and structural adjustability.21,22 Recently, researchers
have spent a great deal of effort in the development of actinide
MOFs and achieved fruitful research results.12,23−26 Hong et al.
Received: December 14, 2019
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tetrakis(4-carboxyphenyl)benzene to synthesize and character-
ize three isomorphous neptunium-based MOFs, which have
rare neptunium oxide wheel clusters and shp type topological
structures.28
The uranyl organic framework is the most studied in actinide
MOFs, while thorium MOFs are relatively rare. Therefore, in
this work, we focus on the development of thorium-based
MOFs. In terms of ligand design, considering the advantages of
using triazine-functionalized polycarboxylic ligands to con-
struct MOFs in the formation of a variety of topological
structures and adsorption properties, we chose the semirigid
triazine hexacarboxylic acid ligand H6TTHA (Scheme 1).29−32
hexapodal ligand H6TTHA. In addition, despite the fact that
there are many structural reports about An frameworks, the
properties of An-MOFs have not been fully explored to a large
extent.37 Herein, we successfully synthesized a novel three-
dimensional (3D) thorium-based Th-TTHA by applying the
Th(IV) cation and semirigid triazine hexacarboxylic acid ligand
H6TTHA and studied its adsorption of iodine in cyclohexane,
which was the first Th-based MOF formed by semirigid
hexapod ligand H6TTHA.
EXPERIMENTAL SECTION
■
Caution! Thorium is a kind of radioactive and chemical toxic element,
which must be treated in strict accordance with the standard precautions
of radioactive substances.
Scheme 1. Structure of Ligand H6TTHA
Synthesis of Ligand 1,3,5-Triazine-2,4,6-triamine Hexa-
acetic Acid (H6TTHA). The 1,3,5-triazine-2,4,6-triamine hexaacetic
acid (H6TTHA) was prepared on the basis of the methods in the
literature (Scheme 2).37,44 Iminodiacetic acid (12.64 g, 95 mmol) and
NaOH (12.00 g, 300 mmol) were placed in 40 mL of distilled water.
Cyanuric chloride (5.53 g, 30 mmol) was dissolved in 40 mL of
deionized water, and then the mixture was added to the solution
described above drop by drop at 0−5 °C. The solution described
above was stirred for 1 h at 0−5 °C, stirred for 3 h at room
temperature, and then refluxed for 12 h at 110 °C. The product was
filtered to obtain a white solid powder, washed with ethanol and
distilled water, and dried at 60 °C, and the final yield was 9.96 g
(70%). Elemental analysis (%) calcd for C15H18N6O12: C, 37.95; H,
3.79; N, 17.71. Found: C, 37.90; H, 3.81; N, 17.73. IR data (KBr,
cm−1): 3469, 2981, 2938, 1722, 1560, 1492, 1397, 1325, 1233, 987,
816, 707, 621, 546.
P r e p a r a t i o n o f T h - T T H A T h 6 O 4 ( O H ) 4 ( H 2 O ) 6 -
(H2TTHA)2(HCO2)4. Thorium nitrate hexahydrate (60 mg),
H6TTHA (24 mg), H2O (1 mL), and DMF (4 mL) were mixed
and stirred for 1 h at room temperature. Subsequently, 4 mol/L nitric
acid was placed drop by drop to adjust the pH of the solution to 3.0,
and the obtained colorless liquid was transferred to a 20 mL Teflon-
lined stainless steel autoclave and heated in an oven at 160 °C for 3
days. Via natural cooling, it was filtered and washed with DMF to
obtain colorless block crystals (52% yield). Elemental analysis (%)
calcd: C, 14.87; , H, 1.46, N, 6.12. Found: C, 15.11; H, 1.41; N, 6.05.
IR data (KBr, cm−1): 3530, 2954, 2794, 1530, 1480, 1421, 1381,
1287, 1175, 1119, 986, 886, 728. Scheme 3 is the synthetic route of
Th-TTHA.
Determination of the Crystal Structure of Th-TTHA by X-ray
Diffraction. A single crystal having a size suitable for Th-TTHA was
glued to a glass fiber to perform an X-ray structure measurement. The
X-ray diffraction data were determined on a Bruker AXS SMART
APEXII CCD diffractometer at room temperature with graphite
monochromatic Mo Kα radiation (λ = 0.71073 Å). The method of
semiempirical absorption correction was used via the SADABS
program.45 The crystal structure of Th-TTHA was determined by
applying the direct method and refined by applying the SHELX-97
program through the full-matrix least-squares method on F2.46,47
Anisotropic displacement parameters were used to refine all non-
hydrogen atoms. The contribution of guest molecules such as H2O
and DMF in the MOF pore to the scattering was calculated by the
PLATON/SQUEEZE method, and a group of guest-free diffraction
On one hand, compared with the rigid tricarboxylic acid
ligands, it has six flexible aminodiacetate arms, which may take
a variety of geometric conformations, such as syn−syn, anti−
anti, and syn−anti;33−35 it has six carboxyl groups that may
take a more abundant coordination mode with the central
metal, such as monodentate, bidentate, and monobidentate,
bridging, chelating, etc., resulting in a more diverse topological
structure.36,37 On the other hand, it has abundant adsorption
sites such as nitrogen-rich triazine ring-containing rich
nitrogens, N of NH-, which is advantageous to adsorb specific
guest molecules such as iodine to promote its adsorption.
H6TTHA ligands and transition metal ions, alkali metal ions,
alkaline earth metal ions, rare earth ions, and uranyl ions have
been used to construct many MOFs with diverse topological
structures and have exhibited significant application in the
realms of luminescence, sensing, adsorption, etc.33,35,38−43
Bruce et al. prepared seven homobimetallic or heterobimetallic
complexes using H6TTHA and metal ions such as sodium,
nickel, chromium, rhodium, zinc, and potassium in a one-pot
method, which is the first time heterobimetallic complexes
containing H6TTHA ligands have been correctly character-
ized.39 Wu et al. utilized H6TTHA to construct rare earth
complexes, which have potential applications in the field of
light applications and LEDs (light-emitting diodes).35 In our
previous work, our group reported three uranyl−organic
frameworks constructed by H6TTHA, which have photo-
electric behavior and show excellent fluorescence sensing
performance for nitrobenzene.38 To the best of our knowledge,
to date, there have been no reports about the crystal structure
of thorium-based MOFs constructed by the semirigid
Scheme 2. Synthetic Route of the Ligand H6TTHA
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Scheme 3. Synthetic Route of Th-TTHA
Figure 1. (a) Inorganic building unit. (b and c) Inner core of Th-TTHA. (d) Cluster 1. (e) Cluster 2. For the sake of clarity, the hydrogen atom
has been omitted. Atom color codes: thorium, yellow; oxygen, pink; nitrogen, blue; carbon, teal.
intensities were given. Then, the structures of Th-TTHA were refined
by using the generated data.48 The crystallographic data of Th-TTHA
are listed in Table S1. The selected bond lengths (angstroms) and
angles (degrees) of Th-TTHA are summarized in Tables S1 and S2.
Iodine Adsorption and Release Experiment. Th-TTHA (100
mg) was soaked in a saturated cyclohexane solution of iodine at room
temperature. The remaining concentration of the iodine solution was
determined at the maximum absorbance at a λmax of 521 nm by
ultraviolet−visible (UV−vis) spectrophotometer. Twenty milligrams
of iodine-loaded sample I2@Th-TTHA was soaked in 10 mL of fresh
ethanol at room temperature. The delivery of iodine in ethanol from
I2@Th-TTHA was monitored with a UV−vis spectrophotometer.
coordination water molecules, two TTHA ligands, and four
HCOO− ions (from DMF decomposition). Among them, six
thorium metal ions form a Th6 octahedron, which occupied the
vertex of the octahedron (Figure 1b,c); each face of the
octahedron is capped by μ3-O(H) groups sharing three
thorium metal centers to form the inner core Th6O4(OH)4.
The existence of the μ3-O(H) group satisfies the requirement
of the framework electroneutrality, which is similar to the
structure of thorium complexes with hexanuclear clusters
reported in previous literature.49,50 Each edge of the
octahedron is bridged by the carboxyl groups from the
TTHA ligand and HCOOH, thus forming the thorium oxide
wheel clusters Th6O4(OH)4(CO2)12 (Figure 1a). This thorium
cluster is similar to that of the thorium MOF Th-TATAB we
reported previously,14 as well as the famous UiO-66.51 In
cluster 1 (Figure 1d), Th3 has a nine-coordinate environment,
which is coordinated with four carboxyl oxygen atoms
originating from two distinct TTHA ligands, four oxygen
atoms that belonged to the μ3-O(H) group, and one atom
from the coordinated water molecule; Th4 is also bound by
nine oxygen atoms, four of which are from the μ3-O(H) group,
two carboxyl oxygen atoms supplied by two distinct TTHA
ligands, and one atom of a coordinated water molecule. Th3
and Th4 are bridged by carboxyl oxygen atoms of two
RESULTS AND DISCUSSION
■
Crystal Structure. The crystal of Th-TTHA (Figure S1)
was obtained under the solvothermal reaction of semirigid
hexapodal linker H6TTHA and Th(NO3)4·6H2O in the
presence of H2O, N,N-dimethylformamide, and nitric acid.
Structural analysis of the X-ray single crystal confirms that Th-
TTHA crystallizes in the tetragonal system in space group I4/
mcm. Its formula, Th6O4(OH)4(H2O)6(H2TTHA)2(HCO2)4,
was determined by the characterization method of CHN
analysis, PXRD, single-crystal X-ray diffraction, IR, UV, and
TG. The molecular structure of Th6O4(OH)4(H2O)6-
(H2TTHA)2(HCO2)4 consists of six thorium(IV) atoms, six
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Figure 2. Structural features of Th-TTHA. (a) Connection mode of the H6TTHA ligand (symmetry codes for #1: −y + 1/2, −x + 1/2, z). (b) Eight
clusters 1 are connected around cluster 2. (c) Eight clusters 2 are connected around cluster 1. (d) Along the b axis, eight clusters 2 are connected
around cluster 1. (e) 3D network structure of Th-TTHA. (f) 3D topological structure of Th-TTHA. For the sake of clarity, the hydrogen atom has
been omitted. Atom color codes: thorium, yellow; oxygen, pink; nitrogen, blue; carbon, teal.
-CH2COOH arms from different N atoms of the same ligand.
In the equatorial direction, two adjacent Th4 atoms are
bridged by carboxyl oxygen atoms from formic acid. In cluster
2 (Figure 1e), Th1 takes the 9-fold environment, which is
connected with four carboxyl oxygen atoms from four distinct
TTHA ligands, four oxygen atoms from the μ3-O(H) group,
and an oxygen atom supplied by H2O in the terminal position.
Th2 also adopts a nine-coordinate environment, which is
linked by two carboxyl oxygen atoms originating from four
distinct TTHA linkers, four oxygen atoms from μ3-O(H), and
one oxygen atom that belonged to the coordinated H2O. Th1
and Th2 are bridged by carboxyl groups of a -CH2COOH arm
from different ligands. In the equatorial direction, the two
adjacent Th2 clusters are bridged by carboxyl oxygen atoms
from formic acid.
As shown in Figure 2a, a TTHA ligand connects three Th
clusters, which itself has a mirror symmetric conformation.
TTHA ligands show great flexibility, in which two
-CH2COOH arms from N and N3#1 bend and deviate above
the plane of the triazine ring to a large extent, ∼68.7° from the
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Figure 3. (a) Thorium oxide wheel cluster of MOF Th-TATAB. (b) Thorium oxide wheel cluster of MOF Th6O4(OH)4(H2O)6(bdc)6·6DMF·
12H2O. (c and d) Two kinds of thorium oxide wheel clusters of Th-TTHA: cluster 1 and cluster 2. (e) Structure of the H6TATAB ligand. (f)
Structure of the terephthalate ligand. (g) Structure of the H6TTHA ligand. For the sake of clarity, the hydrogen atom has been omitted. Atom color
codes: thorium, yellow; oxygen, pink; nitrogen, blue; carbon, teal.
plane of the triazine ring, and connect a cluster 1 by bridging.
The other two -CH2COOH arms from N and N3#1 bend to
below the plane of the triazine ring to a large extent, deviating
from the triazine ring plane ∼65.3° downward, and the
carboxyl oxygen atoms of each -CH2COOH arm bridged a
cluster 2. The whole TTHA ligands connect seven thorium
centers. The specific connection mode can be presented as μ7-
η1η1η1η1η1η1η1η1 (Table S4). The two -CH2COOH arms from
N4 bend to a greater extent below the plane of the triazine
ring, deviating from the plane of the triazine ring nearly 80°
downward, and the carboxyl oxygen atoms of the two
-CH2COOH arms do not participate in the coordination. As
shown in panels b and c of Figure 2, there are eight clusters 1
around cluster 2, and there are eight clusters 2 around cluster
1. From Figure 2d, it can be seen that two carboxyl groups of
two -CH2COOH arms from N and N3#1 are bridged to a
cluster 1, and carboxyl groups of the remaining -CH2COOH
arm of each N atom (N and N3#1) bridged a cluster 2 along
the a axis and bridged the other cluster 2 along the b axis, thus
forming a novel 3D network structure of Th-TTHA (Figure
2e). Figure 2f clearly depicts the 3D topology of Th-TTHA.
Comparison of Coordination Modes of H6TTHA
Ligands with a Central Metal. To date, there have been a
lot of reports of MOFs constructed with the H6TTHA ligand.
Because the H6TTHA ligand has flexible aminodiacetate arms
and six carboxyl functional groups, they often have a variety of
coordination modes. Through a systematic study, it is found
that there are 31 binding modes of the H6TTHA ligand that
have been reported in previous literature (Tables S3 and
S4).33−38,40−42,52−56 Many MOFs assembled from H6TTHA
ligands and transition metal ions, rare earth ions, alkali metal
ions, alkaline earth metal ions, and uranyl ions have been
reported in the literature. The MOFs based on the H6TTHA
ligand and Th(IV) have not been reported. In addition, this
work reports a new connection mode of the H6TTHA ligand
μ7-η1η1η1η1η1η1η1η1 (Table S4), which is different from that
reported elsewhere in the literature.
Comparison of Thorium Oxide Wheel Clusters of a
Thorium-Based MOF Constructed by Triazine Tricar-
boxylic Acid and Terephthalate. The thorium oxide wheel
clusters of Th-TTHA reported in this paper are similar to that
of MOF Th-TATAB (H3TATAB = 4,4′,4″-s-triazine-1,3,5-
triyltri-p-aminobenzoate) previously assembled by Xing’s
research group9 (Figure 3a,e) and Th6O4(OH)4(H2O)6-
(bdc)6·6DMF·12H2O (bdc = terephthalate) reported by
Thierry (Figure 3b,f).49 They all have the same thorium
oxide wheel clusters Th6O4(OH)4(HCO2)12, and each thorium
center is nine-coordinated. However, the sources of carboxyl
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groups in thorium oxide wheel clusters are different. Th-TTHA
has two kinds of thorium oxide wheel clusters: cluster 1 and
cluster 2. In cluster 1, four carboxyl groups are from the
H6TTHA ligand and eight carboxyl groups are from formic
acid (from DMF decomposition). In cluster 2, eight carboxyl
groups are from the H6TTHA ligand and four carboxyl groups
are from formic acid (from DMF decomposition). In the
thorium oxide wheel clusters of MOF Th-TATAB, six carboxyl
groups are from the H3TATAB ligand and six carboxyl groups
are from formic acid (from DMF decomposition). Twelve
carboxyl groups in the thorium oxide wheel clusters of MOF
reported by Thierry are all from terephthalate ligands. In
addition, the structure of thorium oxide wheel clusters is
similar to that of the well-known mofuio-66, which is
constructed by transition metal ions such as tetravalent
zirconium or hafnium.51,57
IR Spectra. The infrared spectrum of the H6TTHA ligand
and the complex Th-TTHA is shown in Figures S4 and S5. For
Th-TTHA, the absorption band at 1530 cm−1 can correspond
to the νCO asymmetric stretching vibrations. The absorption
bands between ∼1400 and ∼1600 cm−1 correspond to the
triazine rings. The absorption peak observed at 1381 cm−1
belongs to νCO symmetric stretching vibrations. The peaks
appearing at 2954 and 2794 cm−1 indicate the presence of
-CH2-. The broad peaks observed near 3400 cm−1 indicate the
appearance of O−H groups from H2O. Upon comparison of
the infrared spectrum of the Th-TTHA complex with that of
the H6TTHA ligand (Figures S4 and S5), one can see that the
asymmetric stretching vibration peak of the Th-TTHA
complex has a red shift of 17 cm−1, from 1547 to 1530
cm−1, while the symmetric stretching vibration peak of the Th-
TTHA complex has a red shift of 16 cm−1, from 1397 to 1381
cm−1, which indicates that the carboxyl oxygen atoms of
H6TTHA have coordinated with the metal centers. Upon
comparison of the infrared spectrum of the iodine-loaded
sample I2@Th-TTHA with Th-TTHA (Figures S5 and S6), it
is obvious that the peak positions of the absorption peak of
I2@Th-TTHA are almost unchanged, but the intensities of the
peaks are dramatically enhanced.
PXRD Analyses. PXRD data of Th-TTHA obtained from
the experimental test are compared with single-crystal
diffraction data simulation (Figure S2). It can be inferred
that Th-TTHA has been gained as pure phases from the
agreement of their peak positions. The preferred orientation of
powder samples may lead to the difference in intensity. In
addition, upon comparison of the powder XRD of the iodine-
loaded sample I2@Th-TTHA and the iodine release sample
with that of complex Th-TTHA, it can be found that the peak
position is basically the same, indicating that the main skeleton
of MOF remains unchanged after iodine adsorption and
release (Figure S2).
Adsorption and Release of Iodine in a Cyclohexane
Solution. First, iodine was dissolved in a cyclohexane solution
to prepare six groups of I2/cyclohexane solutions at
concentrations of 0.1, 0.2, 0.4, 0.6, and 0.8 mg/mL. Using a
cyclohexane reagent as a reference solution, a certain
concentration of the solution was randomly taken, and a full-
wavelength scan was performed with a UV−vis spectropho-
tometer to determine the maximum absorbance at a λmax of
521 nm. Then, the absorbance of the six groups of solutions
described above with different concentrations is measured at a
λmax of 521 nm, to obtain the absorbance curve varying with
concentration (Figure 4). At the same time, the obtained data
Solid UV−Vis Absorbance Spectrum. The solid UV−vis
spectra of both H6TTHA and the Th-TTHA complex were
determined. As one can see from Figure S7, for the ligand
H6TTHA, the appearance of absorption bands at 220, 259, and
310 nm corresponds to the π → π* transition of the ligand; for
Th-TTHA (Figure S8), the appearance of the peaks near 221
and 257 nm can belong to the π → π* transition of the ligand,
and band around 340 nm is associated with LMCT (the
ligand-to-metal transition). It can be clearly seen from the
UV−vis spectrum that the absorption peaks of I2@Th-TTHA
are dramatically broadened compared to those of Th-TTHA
(Figure S9). In addition, the broad peak centered around 350
nm is a characteristic absorption band of a charge transfer
complex involving iodine.58
Figure 4. Calibration plot of iodine in a cyclohexane solution via a
UV−vis spectrum.
are linearly fitted to obtain the calibration plot of iodine in a
cyclohexane solution (Figure 4, inset), and the linear equation
is y = 3.5849x + 0.0057 (R2 = 0.9983). At room temperature,
100 mg of the Th-TTHA sample was soaked in the saturated
cyclohexane solution of iodine. After 0, 1, 3, 6, 9, and 24 h, the
supernatant was taken from the suspension with a syringe and
diluted for analysis as a sample. The residual concentration of
the iodine solution was monitored at the specific time with a
UV−vis spectrophotometer to obtain the adsorption amount
and removal efficiency of TTHA for iodine. The formula for
iodine removal efficiency can be described as
TG Analysis. The analysis of thermogravimetric experi-
ments of Th-TTHA demonstrates that the framework is
thermally stable until 200 °C under N2 (Figure S3). The
weight loss that occurred before 200 °C (∼26%) can be
assigned to the elimination of the coordinated H2O and guest
molecules such as DMF and H2O in MOF channels. After 200
°C, it can be observed that the skeleton begins to collapse, and
finally, the relative plateau is attributed to thorium oxide
(ThO2).
% removal = [(C0 − Ct)/C0] × 100
where C0 represents the initial concentration of the iodine
solution and Ct represents the concentration of the iodine
solution at time t.
The absorption of iodine by Th-TTHA was determined by
the UV−vis spectrum. Figure 5 shows the absorbance curve
with the change in the residual iodine solution concentration.
It can be seen from Figures 6 and 7 that initially, Th-TTHA
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amount can reach 562 mg. The inset of Figure 5 demonstrates
that the crystal color changes from colorless to black after
iodine absorption, which indicates that iodine is incorporated
into the framework of Th-TTHA to continuously generate an
I2-loaded system in solution.59 Moreover, we further
investigated the influence of contact time on the absorption
of iodine by Th-TTHA in a cyclohexane solution, and the
obtained data were linearly fitted using a pseudo-second-order
kinetic equation (Figure 8). It is obvious that the pseudo-
Figure 5. UV−vis spectra of Th-TTHA adsorbing iodine over time in
a saturated cyclohexane solution of iodine when 100 mg of Th-TTHA
is added.
Figure 8. Plot of the adsorption kinetics of iodine by Th-TTHA in a
saturated cyclohexane solution of iodine.
second-order kinetic equation has a good fit for the saturated
iodine cyclohexane solution, and the R2 value is >0.99. The
pseudo-second-order kinetic model can better demonstrate the
kinetics of iodine adsorption by Th-TTHA in a cyclohexane
solution. The pseudo-second-order equation can be described
as follows:60
Figure 6. Iodine removal efficiency of Th-TTHA in a saturated
cyclohexane solution of iodine when 100 mg of Th-TTHA is added.
t
1
t
=
+
2
Q t
Q e
Q e k2
where Qe (milligrams per gram) is the equilibrium adsorption
amount, Qt (milligrams per gram) is the adsorption amount of
the adsorbent at time t, and k2 represents the pseudo-second-
order rate constant (grams per milligram per minute).
The powder X-ray diffraction peak positions of I2@Th-
TTHA are basically the same as those of the Th-TTHA
complex, indicating that the main skeleton after iodine
adsorption remains unchanged (Figure S2). When the IR
spectrum of Th-TTHA is compared, it can be clearly observed
that the vibrations of the benzene ring and the triazine ring of
the iodine-loaded network are dramatically enhanced (Figure
S5). Compared with the solid UV−vis spectra of Th-TTHA,
the spectra of I2@Th-TTHA exhibit dramatically broader
peaks (Figures S7 and S8). The strong adsorption capacity of
Th-TTHA for iodine in a cyclohexane solution is mainly
attributed to abundant active sites, the high porosity, and the
interaction of charge transfer. The nitrogen-rich triazine ring,
uncoordinated carboxyl oxygen atoms, and N atoms of the
tertiary amine of Th-TTHA networks become more effective
sorption sites, due to the formation of the charge transfer (CT)
complex between Th-TTHA networks and polyiodide
anions.32 This facilitates the interaction of iodine molecules
with these exposed and abundant active sites that are
susceptible to being adsorbed into the channels of the MOF
by applying I−I···N and I−I···π halogen bonds.61 Due to the
Figure 7. Adsorption amounts of Th-TTHA toward iodine in a
saturated cyclohexane solution of iodine when 100 mg of Th-TTHA
is added.
adsorbed iodine relatively fast. Within 3 h, an iodine removal
efficiency of 35.35% was reached by Th-TTHA and the
adsorption amount can reach 371.58 mg; after 6 h, an iodine
removal efficiency of 50.75% was reached by Th-TTHA and
the adsorption amount can reach 528.46 mg. After 24 h, the
adsorption of iodine by Th-TTHA almost reached equilibrium,
the removal efficiency reached 53.81%, and the adsorption
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more abundant, exposed, and accessible adsorption sites, and
the 3D network structure, the ability of Th-TTHA to absorb
iodine far exceeds that of the previously reported two-
dimensional uranyl−organic framework U-TATAB con-
structed by triazine tricarboxylic acid ligand MOF MIL53
without functional amino groups.62
The release of iodine from the iodine-loaded sample I2@Th-
TTHA in fresh ethanol was investigated. Figure 8 is a
calibration plot of iodine determined with a UV−vis
spectrophotometer in an ethanol solution, and the absorption
peaks appearing at 290 and 360 nm can be assigned to
polyiodide anions.63,64 At room temperature, the delivery of
iodine in ethanol from an iodine-loaded sample was
investigated by employing a UV−vis spectrophotometer
(Figure 9). The release of iodine in ethanol increases linearly
Figure 11. Efficiency of the release of iodine in ethanol from I2@Th-
TTHA.
ideal nitrogen adsorption results cannot be realized due to the
compact structure and small pore size of Th-TTHA.
CONCLUSIONS
■
In summary, a distinctive 3D metal−organic framework
Th6O4(OH)4(H2O)6(H2TTHA)2(HCO2)4 was prepared and
characterized. Its reaction conditions, structural characteristics,
and adsorption properties with respect to iodine have been
investigated, which is essential for an in-depth understanding
of thorium chemistry and for the management and disposal of
radionuclides and the potential application of the nuclear fuel
cycle.
Figure 9. Calibration plot of iodine in an ethanol solution by UV−vis
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03639.
■
spectra.
sı
with time; the color of the ethanol solution containing the
iodine-loaded sample I2@Th-TTHA gradually deepened, and
the release rate reached ∼90% after 60 min (Figures 10 and
11), indicating that this release behavior of iodine is controlled
by the interaction of the host and guest.64,65 The experimental
results show that the process of Th-TTHA adsorbing iodine is
reversible. Furthermore, although 18% of the solvent-accessible
volume is predicted by using the PLATON software,48 the
Experimental and characterization data, Figures S1−S8,
and Tables S1−S4 (PDF)
Accession Codes
CCDC 1969054 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Feng-Ying Bai − College of Chemistry and Chemical
Engineering, Liaoning Normal University, Dalian 116029, P. R.
China; Email: baifengying2003@163.com
Yong-Heng Xing − College of Chemistry and Chemical
Engineering, Liaoning Normal University, Dalian 116029, P. R.
China; orcid.org/0000-0002-7550-2262;
Email: xingyongheng2000@163.com
Authors
Na Zhang − College of Chemistry and Chemical Engineering,
Liaoning Normal University, Dalian 116029, P. R. China
Li-Xian Sun − Guangxi Key Laboratory of Information
Materials, Guilin University of Electronic Technology, Guilin
541004, P. R. China
Figure 10. UV−vis spectra of the release of iodine in an ethanol
solution.
H
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with Dual Functions of Water Reduction and Oxidation. Adv. Mater.
2018, 30, 1803401.
Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by grants from the National Natural
Science Foundation of China (21571091) and the Common-
wealth Research Foundation of Liaoning Province in China
(20170055).
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